Diamond has a wide band gap of 5.47 eV at room temperature, and is known as a wide bandgap semiconductor.
Among semiconductors, diamond has extremely high dielectric breakdown electric field strength of 10 MV/cm, and a high-voltage operation can be performed. Moreover, diamond has the highest thermal conductivity among known materials, and has an excellent heat radiation property thereby. Further, diamond has very large carrier mobility and saturated drift velocity, and is suitable for a high speed device.
Accordingly, diamond has the highest Johnson performance index, which indicates a property as a radio-frequency and high power device, compared to semiconductors such as silicon carbide and gallium nitride, and is said to be an ultimate semiconductor thereby.
As described above, diamond is expected to be practically used as a semiconductor. It has been desired to supply a large-area and high-quality diamond substrate. However, a diamond substrate with sufficient quality has not been obtained yet.
Currently, Ib type diamond synthesized by a High Pressure and High Temperature (HPHT) method is used as one of a diamond substrate. This Ib type diamond, however, contains many nitrogen impurities, and can only be obtained in a size of about 8 mm square at maximum, thereby being less practical.
Non-Patent Document 1 produces a Schottky diode by using diamond synthesized by a HPTP method as a substrate. This diamond substrate, however, has an etch-pit density by hydrogen plasma treatment, which is a measure of a dislocation defect density, of approximately 105 cm−2. This is reported to be causing operation failure when it is actually formed into an electrode and tried to operate since it contains a killer defect in the vicinity of the electrode or a current path.
In some cases, a homoepitaxial diamond, which was epitaxially grown by a Chemical Vapor Deposition (CVD) method using a diamond synthesized by a HPHT method as a foundation, is used as a substrate.
Non-Patent document 2 tried to produce a mosaic diamond substrate in which plural of diamond substrates are bound in order to obtain a large diamond substrate. Such an art, however, is liable to generate cracks along with the boundary of the substrate.
On the other hand, it has been investigated to produce a diamond substrate by heteroepitaxial growth, in which diamond is grown on the other material. By heteroepitaxial growth, it is possible to obtain a large-area substrate relatively easily, and to reduce the production cost.
As a foundation for heteroepitaxial growth of diamond, silicon (Si), platinum (Pt), etc. have been investigated previously. Non-Patent Document 3 reported that iridium (Ir) is suitable for the foundation material. This is an art to use Ir epitaxially grown on the surface of a single crystal magnesium oxide (MgO) as a foundation material, to subject the surface to treatment for generating a diamond nuclei, and to produce epitaxial diamond by a direct current plasma CVD method.
In heteroepitaxial growth, however, many dislocation defects are generated due to lattice mismatch between diamond and the foundation material. For example, there is large lattice mismatch of 7% between diamond (lattice parameter: 3.57 Å) and Ir (lattice parameter: 3.84 Å). Non-Patent Document 4 reports that an etch-pit density of heteroepitaxial diamond rises to 108 cm−2.
As a method for reducing such a dislocation defect, Non-Patent Document 5 proposes an art called selective growth process. This is a process of lateral growth of diamond from diamond nuclei formed in arbitrary pattern (ELO: Epitaxial Lateral Overgrowth).